1. Field of Invention
The present invention relates to etch chemistries and more particularly to a method for utilizing improved etch chemistries to etch high aspect ratio features.
2. Discussion of the Background
In the semiconductor industry there is a demand for increasing the speed of integrated circuits in general, and memory devices in particular. These demands force semiconductor manufacturers to make devices smaller and smaller on a surface of a semiconductor wafer. Accordingly, the increasingly small top critical dimensions (CDs) used to manufacture semiconductor chips require deep trench capacitors, which are used to store charge for memory, that must go deeper into the silicon to retain the same internal surface area, and, hence, the same capacitance. These smaller top CDs make it more difficult for etchant species to enter such deeply etched holes during etching and for effluent species to be exhausted from the feature as it etches deeper into the silicon wafer. This interaction results in much slower etching and therefore longer etch times, which in turn results in problems providing a mask that will survive the increasingly long etch times. Current technology is approaching a limit of the current masks to be effective with the current chemistry for deep trench etching.
Current deep trench etch chemistries rely on the dissociation of HBr with fluorine-containing gases and O2. By-products of these reactions include SiO2, HF, and other Si—F—Br species. Bromine etches silicon surfaces in a reaction that has a strong physical component in addition to a chemical component. This physical component implies silicon is etched more readily where there exists a direct line-of-sight between the silicon etching surface and the plasma. Silicon dioxide (SiO2), produced by the chemical interaction of silicon from the etched surface and oxygen (O2) from the plasma, forms on the vertical surfaces of the aforementioned trenches where there exists a lesser physical component for etching. In general, process manipulation of SiO2 formation is utilized to control the profile and width of the final trenches.
However, several limitations have been cited for deep trench etch pertaining to the current etch chemistry. For example, mask erosion, particularly near the feature entrance, has posed a formidable issue with current processes. At present, the mask erosion is attributed to chemical attack by HF that is formed by interactions between HBr and NF3. Additionally, mask erosion is attributed to physical sputtering by high energy ions, and decreasing SiO2 surface protection resulting from low silicon availability from smaller (high aspect ratio) features.
In addition to mask erosion, current process chemistries suffer from low etch rates at depths greater than 5 microns for 0.13 to 0.10 micron features or, more generally, once the feature aspect ratio L/d exceeds 50. Beyond this feature aspect ratio, the silicon etch depends primarily on chemical etch rather than physical etch (see FIG. 1).
And lastly, current process chemistries are unable to control the upper profile and maintain an anisotropic etch with parallel side-walls.
In summary, bromine chemistries, particularly those introduced to the plasma as HBr, create HF as a by-product when used in conjunction with fluorine chemistry. The presence of HF in the processing plasma leads to an aggressive attack of the oxide mask. Therefore, an improved chemistry eliminating hydrogen from the plasma is needed to alleviate the aforementioned shortcomings of current practice.